Electrolyte for micro-arc oxidation and method for dyeing substrate therein

ABSTRACT

Disclosed is an electrolyte for micro-arc oxidation, which includes silicate, a fluoride, an alkali metal hydroxide, a pore modifier selected from at least one of triethanolamine and diethanolamine, and a polar solvent. Also disclosed is a method for dyeing a substrate which includes immersing a metal substrate as an anode into the electrolyte, oxidizing the substrate to form an oxide layer on the substrate, applying a solution containing a dye on the oxide layer, and allowing the dye to adhere on the oxide layer, so as to obtain a half-finished product, and forming a sealing layer on the half-finished product.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Invention Patent Application No. 202110007847.2, filed on Jan. 5, 2021.

FIELD

The disclosure relates to an electrolyte and a method for dyeing a substrate therein, and more particularly to an electrolyte for micro-arc oxidation and a method for dyeing a substrate therein.

BACKGROUND

Portable electronic devices on the market are mainly required to be thin and lightweight. Therefore, light metals, such as aluminium alloys or magnesium alloys, are popular materials for manufacturing casings of portable electronic devices due to their excellent mechanical properties and low specific gravity. In particular, magnesium alloys are valued for their good thermal conductivity and shock resistance.

Casings made from magnesium alloys are usually subjected to a dyeing process to modify the appearance thereof, so as to fulfill the needs of consumers and to diversify the appearance of the products thus made. However, magnesium alloys are highly reactive, and thus, a substrate made of the magnesium alloys is usually subjected to an anodic oxidation surface treatment to form an inert porous film before the substrate is subjected to a dyeing process, so as to prevent the substrate from corrosion that might result from reaction thereof with vapor or other chemicals. During the subsequent dyeing process, the inert porous film formed by the anodic oxidation surface treatment allows more dye to be adhered on the substrate, which results in an improved coloring effect.

Nevertheless, such dye tend to non-uniformly adhere to the inert porous film due to the non-uniform distribution and the non-uniform pore size of the pores formed in the inert porous film by the anodic oxidation surface treatment, resulting in non-uniform coloring of the substrate. Additionally, some pores are too small in size such that the dye would not be effectively infiltrated into the inert porous film, creating problems such as difficulty in coloring and/or an inferior color fixation effect.

SUMMARY

Therefore, an object of the disclosure is to provide an electrolyte for micro-arc oxidation that can alleviate at least one of the drawbacks of the prior art.

Another object of the disclosure is to provide a method for dyeing a substrate that can alleviate at least one of the drawbacks of the prior art.

According to a first aspect of the disclosure, there is provided an electrolyte used for micro-arc oxidation. The electrolyte includes a silicate, a fluoride, an alkali metal hydroxide, a pore modifier, and a polar solvent. The pore modifier is selected from at least one of triethanolamine and diethanolamine.

According to a second aspect of the disclosure, there is provided a method for dyeing a substrate, which includes:

providing a substrate made of magnesium or magnesium alloy;

immersing the substrate as an anode into the abovementioned electrolyte;

oxidizing the substrate to form an oxide layer on the substrate, the oxide layer having a plurality of pores;

applying a solution containing a dye on the oxide layer, and allowing the dye to adhere on the oxide layer and to enter into at least a part of the pores, so as to obtain a half-finished product; and

forming a sealing layer on the half-finished product so that the sealing layer wraps the oxide layer and the dye.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment (s) with reference to the accompanying drawings, of which:

FIG. 1 is a schematic view illustrating a magnesium alloy product made by an embodiment of a method for dyeing a substrate according to the disclosure;

FIG. 2 is a scanning electron microscopic (SEM) image of an oxide layer formed on a magnesium alloy substrate by micro-arc oxidation in an embodiment of an electrolyte according to the disclosure; and

FIG. 3 is an SEN image of an oxide layer formed on a magnesium alloy substrate by micro-arc oxidation in a conventional electrolyte.

DETAILED DESCRIPTION

Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics.

An embodiment of an electrolyte used for micro-arc oxidation according to the disclosure includes a silicate, a fluoride, an alkali metal hydroxide, a pore modifier, and a polar solvent. The pore modifier is selected from at least one of triethanolamine and diethanolamine.

In some embodiments, the silicate is selected from at least one of sodium silicate and potassium silicate.

In some embodiments, the fluoride is selected from at least one of sodium fluoride and potassium fluoride.

In some embodiments, the alkali metal hydroxide is selected from at least one of sodium hydroxide and potassium hydroxide.

In some embodiments, the silicate has a concentration ranging from 20 g/L to 100 g/L, the alkali metal hydroxide has a concentration ranging from 20 g/L to 100 g/L, and the fluoride has a concentration ranging from 2 g/L to 15 g/L.

In some embodiments, the pore modifier has a concentration ranging from 0.1 g/L to 5 g/L. When the electrolyte is used to subject a magnesium alloy substrate to micro-arc oxidization, an oxide layer having a plurality of pores is formed on the magnesium alloy substrate. If the pore modifier has a concentration that is too low, the amount of pores formed in the oxide layer will decrease significantly and the size of the pore will also decrease, such that in a dyeing process, a dye will not infiltrate deeply into the oxide layer, which adversely affects the dyeing effect on the magnesium alloy substrate. On the other hand, if the pore modifier has a concentration that is too high, growth of the oxide layer will be inhibited, and the thus formed oxide layer will have a thickness of less than 10 μm.

In some embodiments, the polar solvent is water (e.g., commercially available water, deionized water, etc.). In some embodiments, the electrolyte has a pH value greater than 7 and not greater than 11. In some embodiments, the polar solvent is deionized water, which can be used to prevent additional impurities from being produced by reduction of excess ions contained in the water during the micro-arc oxidation.

Referring to FIG. 1, the aforementioned electrolyte is hereby applied to an embodiment of a method for dyeing a substrate according to the disclosure, which includes:

providing a substrate made of magnesium or magnesium alloy;

immersing the substrate 1 as an anode into the electrolyte;

oxidizing the substrate 1 to form an oxide layer 2 on the substrate 1, the oxide layer 2 having a plurality of pores 22;

applying a solution containing a dye on the oxide layer 2, and allowing the dye to adhere on the oxide layer 2 and to enter into at least a part of the pores 22, so as to obtain a half-finished product; and

forming a sealing layer 3 on the half-finished product so that the sealing layer 3 wraps the oxide layer 2 and the dye.

In the step of immersing the substrate 1 as an anode into the electrolyte, a carbon cathode is immersed into the electrolyte as well. In the step of oxidizing the substrate 1, the substrate 1 is oxidized inwardly from a surface thereof to form an oxide layer 2 thereon. The oxide layer 2 is made of a metal oxide that is formed by oxidation of magnesium or magnesium alloy. The oxide layer 2 has a main body 21 and a plurality of pores 22 recessed downwardly from atop surface of the main body 21. The pore size of the pores 22 ranges from 10 μm to 25 μm, and the oxide layer 2 has a thickness not less than 10 μm.

In some embodiments, the substrate 1 is oxidized by a micro-arc oxidation. During the micro-arc oxidation, the temperature of the electrolyte is controlled to range between 25° C. and 40° C., the applied voltage is controlled to range between 450 V and 550 V, and the average current density across the substrate 1 is controlled at 2 A/dm². During oxidation of the substrate 1, the substrate 1 serving as the anode and the carbon cathode are immersed into the electrolyte, where a gradually increasing voltage is applied continuously, such that a discharge plasma reaction occurs continuously on a surface of the substrate 1, permitting the magnesium or magnesium alloy on the surface of the substrate 1 to form into metal oxide so as to obtain the oxide layer 2. In some embodiments, oxidation of the substrate 1 is carried out for a period of not less than 10 minutes. The oxide layer 2 has a thickness not less than 10 μm, and the appearance thereof is white.

According to the disclosure, triethanolamine or diethanolamine is added into the electrolyte as the pore modifier, such that the pores 22 produced during micro-arc oxidation of magnesium or magnesium alloy have larger pore sizes and improved uniformity. To be specific, when the substrate 1 as the anode is oxidized, an electrolytic oxidation reaction will take place thereon, such that a plurality of oxygen bubbles are generated near the substrate 1. At the same time, the pore modifier is adsorbed on the substrate 1, and thus, the size of the oxygen bubbles and the adsorption intensity of the oxygen bubbles to the substrate 1 are altered, which further enhance the uniformity of the pores 22 formed in the oxide layer 2. During the electrolytic oxidation reaction, the pore modifier releases hydroxide ions to increase the charge quantity on the substrate 1, thereby generating a micro-arc discharge effect, which has a penetration effect on the oxide layer 2 and which is beneficial for increasing the pore size of the pores 22 in the oxide layer 2.

FIG. 2 is a scanning electron microscope (SEM) image of an oxide layer 2 formed on a magnesium alloy substrate 1 by micro-arc oxidation in the electrolyte of the disclosure, which includes triethanolamine as the pore modifier in addition to silicate, fluoride, alkali metal hydroxide, and water. As shown in FIG. 2, the pores 22 in the oxide layer 2 formed after micro-arc oxidation using the electrolyte of the disclosure have pore sizes mainly ranging between 10 μm and 25 μm, and are evenly distributed in the oxide layer 2. FIG. 3 is an SEM image of an oxide layer formed on a magnesium alloy substrate by micro-arc oxidation in a conventional electrolyte, which merely includes silicate, fluoride, alkali metal hydroxide and water without the pore modifier. As shown in FIG. 3, since the conventional electrolyte does not include the pore modifier, the pore size of the pores formed in the oxide layer ranges mainly between 3 μm and 8 μm, and only a small portion of the pores have pore sizes ranging between 16 μm and 25 μm. In other words, compared with a magnesium alloy substrate treated by a conventional electrolyte for micro-arc oxidation, a magnesium alloy substrate 1 treated by the electrolyte of the disclosure for micro-arc oxidation has an increased pore size and distribution uniformity of the pores 22 in the oxide layer 2, such that more filling space can be provided in the oxide layer 2 for adhering a dye in a subsequent dyeing step, which can improve the uniformity of dyeing. In addition, the color of the oxide layer 2 formed by the electrolyte of the disclosure is white, which is advantageous compared with the grayish-white color of the oxide layer formed by using the conventional electrolyte since the white oxide layer 2 does not affect the color fineness of the dye formed thereon, resulting in a product having excellent color purity.

In this embodiment, the method further includes, after oxidizing the substrate 1, cleaning the substrate 1 and the oxide layer 2 so as to remove residue of the electrolyte. In some embodiments, the substrate 1 and the oxide layer 2 are cleaned with water.

The dye is adhered on the oxide layer 2 by applying a solution containing the dye on the main body 21 of the oxide layer 2, such that the dye adheres on the main body 21 and enters into at least part of the pores 22, so as to obtain a half-finished product. In some embodiments, water cleaning can be conducted as needed after the dye is completely adhered, in order to remove residue of the solution or the dye that is not adhered on the oxide layer 2, so that the coloring of the dye on the oxide layer 2 is more uniform. In addition, since the oxide layer 2 has a thickness not less than 10 μm, the intensity of dyeing of the oxide layer 2 is thus increased. That is to say, since the oxide layer 2 formed by the aforementioned micro-arc oxidization has a thickness not less than 10 μm, and also has the pores with high uniformity in position and size distribution, the substrate 1, after adherence of the dye, will have an appearance that is more uniform in color, and occurrence of non-uniform adherence of the dye may be reduced.

The half-finished product is sealed by forming a sealing layer 3 thereon so that the sealing layer 3 wraps the oxide layer 2 and the dye of the half-finished product. In this embodiment, the sealing layer 3 is formed by contacting the half-finished product with a sealing solution containing inorganic salt, which has a temperature controlled to range between 25° C. and 35° C., such that the inorganic salt in the sealing solution is deposited to fill the pores 22 in the oxide layer 2, so as to form the sealing layer 3 on the oxide layer 2, thereby obtaining a magnesium alloy product as shown in FIG. 1. In this embodiment, the inorganic salt in the sealing solution is nickel acetate, but is not limited thereto. By wrapping the dye in the sealing layer 3 formed by the sealing process, color fastness of the dyed magnesium alloy product is further improved.

It should be noted that in addition to use of the sealing solution containing the inorganic salt, the sealing layer 3 may be formed by organic material sealing, moderate temperature sealing, high temperature sealing, or other sealing techniques.

In some embodiments, after forming the sealing layer 3, a cleaning and drying procedure can be conducted as needed to remove residue of the sealing solution.

Examples of the disclosure will be described hereinafter. It is to be understood that these examples are exemplary and explanatory and should not be construed as a limitation to the disclosure.

Example 1

Sodium silicate having a concentration of 30 g/L, potassium fluoride having a concentration of 8 g/L and triethanolamine having a concentration of 0.5 g/L were sequentially added into deionized water, which was used as a solvent, and were mixed well by stirring to obtain a solution. Potassium hydroxide was then added into the solution to adjust the pH value of the solution to 8.5, so as to obtain an electrolyte.

A substrate 1 made of AZ31B magnesium alloy was prepared for oxidation. The substrate 1 as the anode and a carbon cathode were immersed into the electrolyte, and a voltage of 450 V was applied to generate an electric current so as to conduct micro-arc oxidation. Oxidization of the substrate 1 was carried out for a period of 10 minutes using an average current density of 2 A/dm² across the substrate 1. The substrate 1 was oxidized inwardly from its surface to form an oxide layer 2 thereon, the oxide layer 2 being made of metal oxide formed from oxidation of the magnesium alloy. The oxide layer 2 was white in color, and had a thickness ranging between 10 μm and 15 μm. A plurality of pores 22 were formed in the oxide layer 2, and the pore size of the pores 22 ranged from 10 μm to 25 μm. The uniformity of the location distribution of the pores 22 was 53%

$\left( {{{uniformity} = {\frac{1 - {{D_{MAX} - D_{\min}}}}{D_{avg}} \times 100\%}},} \right.$

wherein D_(MAX) is the maximum pore size of the pores 22 in a specific region, D_(min) is the minimum pore size of the pores 22 in the specific region, and D_(avg) is the average pore size of the pores 22 in the specific region), and the porosity of the oxide layer 2 was 41%.

Subsequently, the substrate 1 formed with the oxide layer 2 was cleaned by washing with deionized water to avoid residue of the electrolyte remaining on the oxide layer 2 and on the substrate 1.

Next, the substrate 1 formed with the oxide layer 2 was immersed into a solution containing blue dye (TAC BLUE-RCD, Okuno Chemical Industries Co., Ltd.) having a concentration of 5 g/L, so as to dye the substrate 1 formed with the oxide layer 2 by virtue of dip coating. The substrate 1 formed with the oxide layer 2 was dyed for 17 minutes at a solution temperature of 55° C. such that the blue dye was adsorbed on the main body 21 of the oxide layer 2 and entered into the pores 22. Pure water was then employed to clean the substrate 1 twice to make sure that residue of the dye was removed, thereby obtaining a half-finished product.

Finally, the half-finished product was immersing into a sealing solution (Top Seal DX-500, Okuno Chemical Industries Co., Ltd.) having a concentration of 10 g/L and a temperature of 95° C. for a period of 180 seconds, such that a sealing layer 3 was formed on the oxide layer 2 to seal the half-finished product. Residual sealing solution was then removed by a washing procedure, in which the sealing layer 3 was sequentially washed twice with water having an ambient temperature and once with hot water having a temperature of 65° C. A drying procedure was then conducted at a drying temperature of 100° C. for 600 seconds, thereby obtaining a magnesium alloy product with a light blue appearance.

Example 2

An electrolyte similar to that in Example 1 was provided, and a substrate 1 made of AZ31B magnesium alloy was prepared for oxidation.

The procedures and conditions for oxidizing the substrate 1 were substantially the same as those in Example 1 except that the applied voltage was 465 V. The oxide layer 2 thus obtained has a white color, and the thickness thereof ranged from 10 μm to 15 μm. A plurality of pores 22 were formed in the oxide layer 2. The pore size of the pores 22 ranged from 10 μm to 25 μm, the uniformity of position distribution of the pores 22 was 53%, and the porosity of the oxide layer 2 was 41%. Subsequently, the substrate 1 formed with the oxide layer 2 was cleaned by washing with deionized water to avoid residue of the electrolyte remaining on the oxide layer 2 and the substrate 1.

Next, the substrate 1 formed with the oxide layer 2 was immersed into a solution containing blue dye (TAC BLUE-RCD, Okuno Chemical Industries Co., Ltd.) having a concentration of 5 g/L and a temperature of 55° C. for a period of 10 minutes, so that the substrate 1 is dyed. The blue dye was adsorbed on the main body 21 of the oxide layer 2 and entered into at least part of the pores 22. Pure water was then employed to clean the substrate 1 twice to make sure that residue of the dye was removed from the oxide layer 2, thereby obtaining a half-finished product. Finally, the half-finished product was sealed by forming a sealing layer 3 according to the procedures described in Example 1, so as to obtain a magnesium alloy product with a blue appearance.

Example 3

Sodium silicate having a concentration of 35 g/L, potassium fluoride having a concentration of 7.5 g/L, and triethanolamine having a concentration of 1 g/L were sequentially added into deionized water that serves as a solvent, and then were mixed by stirring to obtain a solution. Then, the pH value of the solution was adjusted to 8.5 by adding potassium hydroxide, thereby obtaining an electrolyte.

A substrate 1 made of AZ31B magnesium alloy was prepared for oxidation. The substrate 1 as an anode and a carbon cathode were immersed into the electrolyte, and a voltage of 550 V was applied to generate an electric current so as to conduct micro-arc oxidation that was carried out for a period of 10 minutes using an average current density of 2 A/dm² across the substrate 1. The substrate 1 was oxidized inwardly from its surface to form an oxide layer 2 thereon, the oxide layer 2 being made of metal oxide formed by oxidation of magnesium alloy. The oxide layer 2 was white in color, and had a thickness ranging between 12 μm and 16 μm. A plurality of pores 22 were formed in the oxide layer 2. The pore size of the pores 22 ranged from 10 μm to 25 μm, the uniformity of the position distribution of the pores 22 was 50%, and the porosity of the oxide layer 2 was 37%.

Subsequently, the substrate 1 formed with the oxide layer 2 was cleaned by washing with deionized water to avoid residue of the electrolyte remaining on the oxide layer 2 and the substrate 1.

Next, the substrate 1 formed with the oxide layer 2 was dyed in a manner similar to that in Example 1. The substrate 1 formed with the oxide layer 2 was immersed into a solution containing blue dye (TAC BLUE-RCD, Okuno Chemical Industries Co., Ltd.) having a concentration of 5 g/L. The substrate 1 was dyed for a period of 10 minutes at a solution temperature of 55° C. The blue dye was adsorbed on the main body 21 of the oxide layer 2, and entered into at least part of the pores 22. Pure water was then employed to clean the substrate 1 twice to make sure that residue of the dye remaining on the oxide layer 2 and the substrate 1 was removed, thereby obtaining a half-finished product.

Finally, the half-finished product was sealed by immersing into a sealing solution (Top Seal DX-500, Okunc Chemical Industries Co., Ltd.) having a concentration of 10 g/L, such that a sealing layer 3 was formed to wrap the blue dye and the oxide layer 2 of the half-finished product. The sealing procedure for forming the sealing layer 3 was the same as that in Example 1. A magnesium alloy product with a dark blue appearance was thus obtained.

Example 4

An electrolyte similar to that in Example 3 was provided, and a substrate 1 made of AZ31B magnesium alloy was prepared for oxidation.

The procedures and conditions for oxidization of the substrate 1 were similar to those of Example 3. The substrate 1 was oxidized by immersing the substrate 1 as an anode into the electrolyte, so as to form an oxide layer 2 on the substrate 1. The oxide layer 2 was made of oxidized metal formed from oxidation of magnesium alloy. The oxide layer 2 was white in color, and had a thickness ranging from 12 μm to 16 μm. A plurality of pores 22 were formed in the oxide layer 2. The pore size of the pores 22 ranged from 10 μm to 25 μm, uniformity of the position distribution of the pores 22 was 48%, and the porosity of the oxide layer 2 was 42%.

Subsequently, the substrate 1 formed with the oxide layer 2 was cleaned by washing using deionized water, so as to avoid residue of the electrolyte remaining on the oxide layer 2 and on the substrate 1.

Next, the substrate 1 formed with the oxide layer 2 was immersed into a solution containing brown dye (TAC BROWN-GR, Okuno Chemical Industries Co., Ltd.) which has a concentration of 5 g/L. The oxide layer 2 was dyed by dip coating. The substrate 1 formed with the oxide layer 2 was dyed for a period of 8 minutes at a solution temperature of 55° C. The brown dye was adhered on the main body 21 of the oxide layer 2, and entered into at least part of the pores 22. Then, pure water was employed to clean the substrate 1 twice to make sure that residue of the dye was removed from the oxide layer 2, thereby obtaining a half-finished product.

Finally, the half-finished product was sealed by forming a sealing layer 3 using procedures similar to those of Example 1, thereby obtaining a magnesium alloy product with a brown appearance.

Example 5

An electrolyte similar to that in Example 3 was provided, and a substrate 1 made of AZ31B magnesium alloy was prepared for oxidation. The substrate 1 as the anode was immersed into the electrolyte to conduct the oxidation as described in Example 3, so as to form an oxide layer 2 on the substrate 1. The oxide layer 2 was made of metal oxide formed by oxidation of magnesium alloy. The oxide layer 2 was white in color, and had a thickness ranging between 12 μm and 16 μm. A plurality of pores 22 were formed in the oxide layer 2. The pore size of the pores 22 ranged from 10 μm to 25 μm, the uniformity of the position distribution of the pores 22 was 52%, and the porosity of the oxide layer 2 was 39%. Subsequently, the substrate 1 formed with the oxide layer 2 was cleaned by washing with deionized water to avoid residue of the electrolyte remaining on the oxide layer 2 and on the substrate 1.

Next, the substrate 1 formed with the oxide layer 2 was immersed into a solution containing green dye (TAC Green-GM(1), Okuno Chemical Industries Co., Ltd.) which has a concentration of 5 g/L, and the oxide layer 2 was dyed by dip coating. The substrate 1 formed with the oxide layer 2 was dyed for a period of 10 minutes at a solution temperature of 55° C. The green dye was adhered on the main body 21 of the oxide layer 2, and entered into at least part of the pores 22. Pure water was then employed to clean the substrate 1 twice to make sure that residue of the dye was removed from the oxide layer 2, thereby obtaining a half-finished product.

Finally, the half-finished product was sealed for a period of 60 seconds such that a sealing layer 3 was formed thereon, thereby obtaining a magnesium alloy product with an army green appearance.

Comparative Example 1

Sodium silicate having a concentration of 55 g/L and potassium fluoride having a concentration of 2 g/L were sequentially added into deionized water that was used as a solvent, and then were mixed by stirring, so as to obtain a solution. Sodium hydroxide was then added into the solution to adjust pH value of the solution to 8.5, so as to obtain an electrolyte.

Next, a substrate made of AZ31B magnesium alloy was prepared for oxidation. The substrate as an anode and a carbon cathode were immersed into the electrolyte that was not added with the pore modifier. A voltage of 500 V was then applied to generate an electric current, and to control the average current density of 2 A/dm² across the substrate. The substrate was oxidized for a period of 10 minutes, i.e., the substrate was subjected to micro-arc oxidation inwardly from its surface so as to form an oxide layer on the substrate. The oxide layer was grayish-white in color, and had a thickness ranging between 4 μm and 10 μm. A plurality of pores were formed in the oxide layer. The pore size of the pores ranged from 5 μm to 15 μm, the uniformity of the position distribution of the pores was 10%, and the porosity of the oxide layer was 8%.

Evaluation of Color Value

The color value for each of the magnesium alloy products obtained in Examples 1 to 5 and Comparative Example 1 was performed using a spectrophotometer and the color values were determined based on the coordinates of the CIELAB color space. The color values L*, a* and b* were obtained, wherein the color value L* represents the perceptual lightness of the color (the higher the value, the lighter the color is), the color value a* represents the green-red chromaticity coordinate (negative values indicate green and positive values indicate red), and the color value b* represents the blue-yellow chromaticity coordinate (negative values indicate blue and positive values indicate yellow).

The composition (except water) of the electrolytes used in Examples 1 to 5 and Comparative Example 1, and the color value evaluation results of the magnesium alloy products obtained in Examples 1 to 5 and Comparative Example 1 are summarized in Table 1.

TABLE 1 Comparative Examples Example 1 2 3 4 5 1 Electrolyte Sodium 30 30 35 35 35 55 silicate (g/L) Potassium 8 8 7.5 7.5 7.5 2 fluoride (g/L) Triethanol- 0.5 0.5 1.0 1.0 1.0 — amine (g/L) Oxidation time (min.) 10 10 10 10 10 10 Voltage(V) 450 465 550 550 550 500 Dye TAC TAC TAC TAC TAC — BLUE-RCD BLUE-RCD BLUE-RCD BROWN-GR Green-GM (blue) (blue) (blue) (brown) (green) Dyeing temperature(° C.) 55 55 55 55 55 — Dyeing period (min.) 17 10 10 8 10 — Sealing temperature (° C.) 95 95 95 95 95 — Sealing time (s) 180 180 180 180 60 — Appearance Oxide Thickness (μm) 10~15 10~15 12~16 12~16 12~16 4~10 layer Pore size (μm) 10~25 10~25 10~25 10~25 10~25 5~15 Pore uniformity (%) 53% 53% 50% 48% 52% 10% Porosity (%) 41% 41% 37% 42% 39%  8% Color of magnesium Light Blue Dark Brown Army Grayish- alloy product blue blue green white Color L* 55~57 46~52 40~45 55~57 70~80 78~85  value a* −3.89  −1.77  2.69 9.47 −23.80 −0.11 b* −16.33 −20.53 −24.14 7.62 62.11 0.92

Results in Table 1 show that the magnesium alloy products of Examples 1 to 5, which were obtained by oxidizing the substrates using the electrolyte of the disclosure, all have an oxide layer 2 having a thickness not less than 10 μm. The porosity of the oxide layer 2 ranges from 37% to 42%, the pore size of the pores 22 ranges from 10 μm and 25 μm, and the uniformity of the pores 22 ranges from 48% to 53%. Compared with the magnesium alloy product of Comparative Example 1, which was obtained by subjecting the magnesium alloy substrate to micro-arc oxidation using a conventional electrolyte for the same oxidation period, it is shown that by using the electrolyte of the disclosure, the thickness of the oxide layer 2, the pore size of the pores 22, and the uniformity of the distribution of the pores 22 can be effectively increased.

In addition, the results show that the period needed for dyeing the oxide layer 2 formed on the magnesium alloy substrate 1 is no more than 20 minutes, i.e., to have the dye adsorbed effectively on the main body 21 and filled in the pores 22, indicating that a favorable dyeing effect is achieved, as shown by an increase in the pore size and the distribution uniformity of the pores 22 formed in the oxide layer 2 of the magnesium alloy substrate 1 made by micro-arc oxidization using the electrolyte of the disclosure.

In summary, by including the pore modifier in the electrolyte of the disclosure, the pore size of the pores 22 in the oxide layer 2 formed by micro-arc oxidation and the porosity of the oxide layer 2 can be increased, which provides the dye with sufficient adsorption area and filling space, and thus, the dyeability of the oxide layer 2 can be increased. Since the distribution uniformity of the pore size and the position of the pores 22 are enhanced, the color uniformity of the dyed oxide layer 2 can be improved. Additionally, the color durability can be further prolonged by wrapping the dye and the oxide layer 2 in the sealing layer 3.

In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiment(s). It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, FIG., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure.

While the disclosure has been described in connection with what is (are) considered the exemplary embodiment(s), it is understood that this disclosure is not limited to the disclosed embodiment(s) but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements. 

What is claimed is:
 1. An electrolyte used for micro-arc oxidation, comprising: a silicate; a fluoride; an alkali metal hydroxide; a pore modifier selected from at least one of triethanolamine and diethanolamine; and a polar solvent.
 2. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said pore modifier has a concentration ranging between 0.1 g/L and 5 g/L.
 3. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said silicate is selected from at least one of sodium silicate and potassium silicate.
 4. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said fluoride is selected from at least one of sodium fluoride and potassium fluoride.
 5. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said alkali metal hydroxide is selected from at least one of sodium hydroxide and potassium hydroxide.
 6. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said silicate is selected from at least one of sodium silicate and potassium silicate, said fluoride is selected from at least one of sodium fluoride and potassium fluoride, and said alkali metal hydroxide is selected from at least one of sodium hydroxide and potassium hydroxide.
 7. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said polar solvent is water.
 8. The electrolyte used for micro-arc oxidation as claimed in claim 1, wherein said electrolyte has a pH value greater than 7 and not greater than
 11. 9. A method for dyeing a substrate, comprising: providing a substrate made of magnesium or magnesium alloy; immersing the substrate as an anode into the electrolyte as claimed in claim 1; oxidizing the substrate to form an oxide layer on the substrate, the oxide layer having a plurality of pores; applying a solution containing a dye on the oxide layer, and allowing the dye to adhere on the oxide layer and to enter into at least a part of the pores, so as to obtain a half-finished product; and forming a sealing layer on the half-finished product so that the sealing layer wraps the oxide layer and the dye.
 10. The method as claimed in claim 9, wherein oxidizing the substrate is conducted by micro-arc oxidation.
 11. The method as claimed in claim 10, wherein oxidizing the substrate is conducted in a voltage ranging between 450 V and 550 V, and an average current density across the substrate is 2 A/dm².
 12. The method as claimed in claim 9, wherein in oxidizing the substrate, the electrolyte is maintained at a temperature ranging between 25° C. and 40° C.
 13. The method as claimed in claim 9, wherein oxidizing the substrate is carried out for a period not less than 10 minutes.
 14. The method as claimed in claim 9, wherein the oxide layer has a thickness not less than 10 μm, and the appearance of the oxide layer is white.
 15. The method as claimed in claim 9, wherein the pores have a pore size ranging from 10 μm to 25 μm.
 16. The method as claimed in claim 9, further comprising, after oxidizing the substrate, cleaning the substrate and the oxide layer so as to remove residue of the electrolyte.
 17. The method as claimed in claim 9, wherein forming the sealing layer is conducted by contacting the half-finished product with a sealing solution that contains nickel acetate at a temperature ranging from 25° C. to 35° C. 